More recently, it has been proposed to use for these controls touch-sensitive surfaces making it possible to detect a simple pressure of the driver's finger and, depending on the position of the detected pressure and/or on the subsequent movement of this pressure on the surface, to initiate a particular type of action or of control of a member of the motor vehicle. Reference may be made, for example, to documents FR 2 798 329, FR 2 800 885 and U.S. Pat. No. 6,157,372. These touch-sensitive surfaces may be of any type and use various technologies.
Therefore the technology using pressure-sensitive resistors (also known as FSR for “Force Sensing Resistor” sensor) is increasingly ahead of other equivalent technologies, such as for example capacitive or else optical technologies, by virtue of its ease of application and its robustness.
Such sensors are, for example, known by the name “digitizer pad” and documents U.S. Pat. No. 4,810,992, U.S. Pat. No. 5,008,497, FR 2683649 or else EP 0 541 102 are cited as the prior art.
These sensors comprise semiconductive layers sandwiched between, for example, a conductive layer and a resistive layer. By applying a pressure to the FSR layer, the ohmic resistance diminishes, thereby making it possible, by application of an appropriate voltage, to measure the pressure applied and/or to locate the place where the pressure is applied.
The touch-sensitive surface of FSR sensors has a flat shape that can be curved only very slightly in one dimension, which prevents any three-dimensional production.
To improve the user-friendliness of motor vehicles and in particular for control switches or keypads, it may be necessary to incorporate the touch-sensitive surface sensors into control devices having a recessed or protruding three-dimensional surface, having, for example, a shape with a greater or lesser curve.
With the current touch-sensitive surface sensors, the radius that can be accepted by the active zone of the sensor is small (the radius of curvature must be greater than 250 mm), which limits the choice of the shape of the control device.
An alternative consists in having a touch-sensitive surface sensor that is flat in the control device and in giving a curved shape to the device by covering the sensor with a skin, such as a silicone skin, the thickness of which is a function of the desired curve.
Therefore, to obtain a control device of curved shape, there is on the sensor a skin which is thicker at the center than at the edges. However, this arrangement reduces the sensitivity of the sensor because the forces are transmitted to the sensor through the thickness of the skin.